In the field of climbing towards or along a wall structure, ropes or cables are used by climbers. Such ropes or cables need to be secured at an anchoring point to an anchoring device firmly attached to the wall structure, i.e. the climber hangs from a rope or cable which in turn hangs from the anchoring device. In traditional climbing along the wall of a mountain, an anchoring device may consist of a pin which is hammered into the mountain wall to provide an anchoring point.
When the wall structure either completely or partially consists of a magnetizable material, such as steel, an anchoring device may comprise a magnet unit which is adapted to generate a magnetic field which causes the anchoring device to attach to the magnetizable material of the wall structure through the development of a magnetic attracting force. A wall structure comprising magnetizable material may be made specially for a specific purpose, e.g. for training of climbing or for entertainment, but can also be a wall structure of e.g. a ship or other vessel or other man-made structure, where an anchoring device is needed at an arbitrary location e.g. for maintenance or repair purposes.
Magnets which generate an attracting force on magnetizable material are generally known in different types, and can be comprised in a magnet unit in an appropriate number, and in an appropriate combination of types, if desired. A first type of magnet is a permanent magnet. A second type of magnet is an electromagnet. A third type of magnet is an electropermanent magnet.
A permanent magnet comprises a magnetic material like ferrite, neodymium (e.g. NdFeB), or other suitable materials, and generates a permanent magnetic field over time. An electromagnet comprises a magnetizable material (e.g. steel) and one or more electric conductors. When a current flows through the conductor(s), the electromagnet generates a magnetic field corresponding to the current. When the current is taken away from the electromagnet the magnetic field reduces to very low magnetic field. An electropermanent magnet comprises a combination of permanent magnetic material and magnetizable material that remains magnetic after magnetizing (e.g. AlNiCo), and one or more electric conductors. When a brief current pulse is applied to flow in the conductor(s) in one direction, the electropermanent magnet generates a magnetic field, whereas when a brief current pulse is applied to flow in the conductor(s) in a reverse direction, the electropermanent magnet generates no, or a low magnetic field. Accordingly, an electropermanent magnet can be switched on and off by applying a pulse of current flowing in a predetermined direction to its conductor(s). For any type of magnet, an additional structure (yoke) of magnetizable material may be combined with the magnet to shape, e.g. guide, focus or redirect, the magnetic field generated by the magnet in a predetermined way at a predetermined location.
It is known e.g. from U.S. Pat. No. 7,052,447 to provide magnet units mountable to a hand or a leg. Each magnet unit comprises one or more permanent magnets. In combination, the magnet units are part of a magnetic climbing system. Each magnet unit is a mobile magnetic anchoring device, i.e. it can be placed and replaced at an anchoring point on a wall structure comprising magnetizable material, where it will be secured to the wall structure by magnetic attracting forces.
FR 2517106 discloses a mobile anchoring device having magnet units attachable to a ceiling. A harness flexibly connects the magnet units which are to be attached to the hands and knees of a person. The contact of each magnet unit with the wall structure is detected by two attachment detectors 44 (FIG. 4) which each comprise a sensing pin 48 slidably mounted in a bore of a housing 45. On the one hand, when the magnet unit is not placed on a surface S, the sensing pin 48 extends to beyond the attachment surface of the magnet unit. On the other hand, when the magnet unit has been placed on the surface, the sensing pin 48 is displaced in the bore of the housing 45 against a force of a spring 47, to close electrical contacts 50. Thus. the attachment detectors 44 establish whether a magnet unit is placed on a wall structure by sensing a sensing pin position.
In order for the magnetic anchoring device not to move relative to the wall structure inadvertently (either by its own weight or by a force directed generally along, or away from the wall structure), the magnetic anchoring device should generate sufficient magnetic attracting force between the magnetic anchoring device and the wall structure. If the magnetic attracting force is insufficient, the magnetic anchoring device may e.g. slide along the wall structure when an external force is applied which has a component along (parallel to) the local surface of the wall structure, and when the frictional force between the magnetic anchoring device and the wall structure is overcome by the external force (the frictional force is proportional to the attracting force between the magnetic anchoring device and the wall structure). Likewise, an external force exerted on the magnetic anchoring device, and having a component away from (at right angles to) the local surface of the wall structure, may detach the magnetic anchoring device from the wall structure if the magnetic attracting force between the magnetic anchoring device and the wall structure is insufficient.
In practice, the surface of a magnetic wall structure will not be perfectly flat, may be damaged and/or may have layers of rust or a surface coating like paint or plastic. Such surface condition may vary from place to place, and provides a variable and unpredictable air gap between the magnetic wall structure and a magnetic anchoring device. The attracting force between the magnetic anchoring device and the wall structure therefore may vary up to the point where a user of a magnetic anchoring device cannot be sure whether a load (force) exerted on the magnetic anchoring device can be supported by the magnetic anchoring device without moving it relative to the wall structure. This may lead to dangerous situations, specifically when people's well-being depends on the reliability of the support provided by the magnetic anchoring device relative to the wall structure.